For decades, scientists have been making predictions about how climate change will hurt health around the world.

But actually showing a link? That's been pretty tough.

Take for example, mosquito-borne diseases. It's easy to blame rising temperatures for the global spread of Zika or the explosion of dengue fever. Mosquitoes thrive in higher temperatures, right?

Yes and no. As we reported earlier this year, warmer weather doesn't necessarily mean mosquitoes are more likely to spread viruses like dengue, yellow fever and Zika. Higher temperatures can actually reduce transmission of viruses because the insect's lifespan can decrease in warmer weather. So the mosquito may die before the virus has time to mature and become infectious inside of it.

In other words, climate's connection to health is extraordinarily complicated.

Now international team of scientists has taken a step toward untangling this problem on a global scale.

"All of the work we present is pretty tricky," says Dr. Nick Watts, at University College London, who led the study. "I don't think any of us would ever say that this has been easy."

Around the world, people have experienced an average increase in temperature about 1.5 degrees Fahrenheit, and the study — published Monday in The Lancet journal — finds several signs that even this small amount of warming threatens the health of hundreds of millions of people each year.

First, the number of vulnerable people exposed to heat waves has surged worldwide, the study finds. In the past few years, more about 125 million people over age 65 experienced heat waves each year, compared to about 19 million people each year in the 1990s.

"That's a pretty stunning number," says Kim Knowlton, an environmental researcher at Columbia University, who studies climate change and health at but wasn't involved in this study. "Heat waves aren't just an inconvenience. Heat kills." And it also exacerbates existing problems, such heart disease and kidney problems.

This recent surge in heat waves is consistent with previous studies looking at health and climate change, including meta-analyses published by the Intergovernmental Panel on Climate Change in 2014. "Rising temperatures have [likely] increased the risk of heat-related death and illness," the IPCC wrote.

The second major consequence of warming temperatures is an increase in weather-related disasters. The frequency of floods, droughts and wildfires, collectively, has increased by 46 percent since the 1980s, rising from about 200 events each year to 300 events per year. And some of that increase is due to climate change, the Lancet study finds.

Families around the world — including those in the U.S. — are already experiencing these events firsthand, Knowlton says.

"Communities are hurting. People are reeling globally," she says. "And I think this experience might mark a turning point in the public's perception of climate change because people are connecting the dots to their health, here and now."

Surprisingly, the study finds that deaths from weather-related disasters has not increased during the same time period.

"There's no discernible upward or downward trend in the lethality of these extreme weather events," Watts says. "That may simply be because the data is not over a long enough period of time to isolate that trend."

And then there's the question of mosquito-borne diseases. Since 1990, annual cases of dengue worldwide have doubled each decade. Much of this rise is likely due to rapid urbanization and global travel, the World Health Organization says.

But Watt and his collaborators do find that climate change has contributed to dengue's explosion — at least a little bit.

"We're not trying to say that allcases of dengue fever are the result of climate change," Watts says. "But we've identified a very strong signal in the climate trends that are increasing the capacity of the Aedes mosquitoes to spread dengue." Although they don't yet know why.

Specifically, the team estimates that climate change has increased dengue transmission by Aedes aegypti and Aedes albopictus by 3 percent and 6 percent, respectively, since 1990.

Finally, the Lancet study also analyzes what countries are doing to slow down climate change. "That's probably the part of this report that really surprised me," Watts says. "There are glimmers of hope in that data."

For the past 25 year, he says, countries have been basically doing very little to reduce carbon emissions. "Progress has been woefully inadequate."

But now there are signs the tide may be turning by a small degree.

"In the past five years, we have started to see an acceleration in the response to climate change," Watts says.

In particular, the use of coal around the world has slowed down, the study finds and even possibly even peaked in 2013. Some countries are relying more on natural gas and some are starting to swap in renewable energy sources — like geothermal, hydropower, ocean energy, solar energy and wind energy — which not only reduce carbon emissions but also make the air healthier for people to breath.

"That's really exciting," Watt says. "Because we could all use a little bit of hope at the moment."

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DAVID GREENE, HOST:

So scientists have been predicting for decades how climate change might hurt people's health. Well, now they say they are beginning to see actual damage. These results come from a large international collaboration with two dozen universities and U.N. agencies. NPR's Michaeleen Doucleff reports.

MICHAELEEN DOUCLEFF, BYLINE: In the past few decades, the average temperature people have experienced around the world has gone up by about 1.5 degrees Fahrenheit. That change may seem small, but Kim Knowlton at Columbia University says it has had two main consequences on health. First off, there's been a spike in heat waves worldwide. The number of vulnerable people exposed has shot up more than six times.

KIM KNOWLTON: In one particular year, 2015, there were 175 million more people exposed to extreme heat. That is not just an inconvenience. Heat kills people.

DOUCLEFF: And it exacerbates existing health problems such as heart disease and kidney failure. Then there's the rise in weather-related disasters. The study found the frequency of floods, droughts and wildfires has increased by almost 50 percent since 2000, and some of that surge is because of climate change. Knowlton says people around the world are experiencing this firsthand.

KNOWLTON: And, you know, people are suffering. Communities are hurting. People are reeling globally. And I think that's the turning point, that people are connecting the dots between climate change and health here and now.

DOUCLEFF: In terms of infectious diseases, climate change often gets blamed for causing the rise of mosquito-borne viruses like Zika, but other factors have likely contributed more to their spread, like urbanization and travel. The study, published in The Lancet journal, found a tiny link between climate change and dengue fever, another mosquito-borne virus, but not a link to any other diseases. The study also looks at what countries are doing to slow down climate change. Nick Watts at the University College London led the study. He says this is the part with a tiny, tiny sliver of hope.

NICK WATTS: That's probably the part of this that really surprised me.

DOUCLEFF: For more than two decades, he says, countries had been basically doing very little to reduce carbon emissions.

WATTS: Taken as a whole, for the last 25 years, we broadly see that progress has been woefully inadequate.

DOUCLEFF: But now there are signs the tide is turning, at least a small amount.

WATTS: Just at the last five years we've started to see an acceleration in the response to climate change, and that's something really exciting because I think we could all use a little bit of hope at the moment.

DOUCLEFF: In particular, Watts says, the use of coal around the world has slowed down and possibly even peaked.

WATTS: And we've seen it decline, and we're continuing to see it decline right the way up to 2016 when our data stops. of coal.

DOUCLEFF: Instead of coal, Watts says some countries are relying more on natural gas and are starting to swap in renewable energy sources, which not only reduce carbon emissions but also make the air healthier to breathe. Michaeleen Doucleff, NPR News. Transcript provided by NPR, Copyright NPR.